The invention relates generally to disc filters. Of particular interest to the invention are a method and arrangement for monitoring or sensing the thickness of a filter cake which builds up in a disc filter, especially a purifying filter of the centrifugal type.
A knowledge of the thickness of a filter cake is of significance in order that too great a build-up of the filter cake and an overloading of the filter discs, which may result in damage to the filter discs or to the entire filter, may be avoided. Although too great a thickness of the filter cake is undesirable for the reason mentioned, too small a thickness is also undesirable since the degree of utilization of the filter than becomes too low.
It is possible to calculate the quantity of solid material which accumulates on a filter cloth. However, when the proportion of solids in the fluid to be filtered varies, large inaccuracies may result and the risk of overloading is not eliminated.
Various methods for determining the filter cake thickness have also been proposed.
An apparatus for monitoring the thickness of a filter cake built on the upper surface of a horizontal filter element is described in the German Federal Republic Pat. No. 1,958,568. In this apparatus, a support arm is mounted on the wall of the filter. This support arm, which is accessible from exteriorly of the filter, may be moved over a filter element by means of a motor. In one embodiment of the apparatus, the support arm has a forked configuration and a light emitter and light detector are arranged at the tips of the times of the support arm.
Such an optical apparatus has the disadvantage that no measurement whatsoever can be obtained if the light transmissivity of the fluid being filtered is too low. Another disadvantage resides in that no contact may be had with alkaline solutions when the casing for the light is composed of glass. On the other hand, when the casing is composed of synthetic resin, the range of organic solvents which may be filtered is restricted. Moreover, apparatus of this type are susceptible to elevated temperatures. A further disadvantage arises because disassembly and removal of the optical apparatus are necessary when the filter is to be cleaned.
It is also known to use measurements of pressure differential as an indicator of the extent to which the filter discs are loaded with filter cake. This system functions satisfactorily when difficult-to-filter media are involved since a large pressure differential is already created with a small filter cake thickness. However, when the media involved are of the type which may be readily filtered, the pressure differentials created are small. Thus, even though overloading has occurred, the difference in pressure may still be very small.
Another apparatus is described in the German Federal Republic Offenlegungsschrift 23 58 365. This apparatus includes a hollow body which becomes plugged by the growing filter cake. The resulting difference in pressure between the interior of the hollow body and the space surrounding the latter is used as an indicator of the filter cake thickness.
The German Democratic Republic Pat. No. 92,700 describes a method, and a corresponding apparatus, for measuring the thicknesses of filter cakes in alluvial filters. Here, the desired filter cake thickness is selected and set. A fluid stream is branched off and the thickness of the filter cake produced therefrom is measured.
None of the above-described measuring methods and apparatus are completely satisfactory since they do not permit optimum operating conditions to be achieved. An overloading of the filter discs or plates occurs time and again due to the shortcomings of the control apparatus. As a result of such overloading of the individual discs, as well as the entire filter package, are deformed or destroyed.
The deformation often manifests itself in that the discs are pushed or bent away from one another or in that the discs are pushed or bent in a predominantly downward direction. The generally accepted explanation for such deformations is that the solids present in the fluid being filtered penetrate the space between two adjacent discs in a wedge-like manner and that the plates are bent by such penetration.